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  • 1
    Electronic Resource
    Electronic Resource
    A small optimal complete active space (CAS) for multiconfigurational spin tensor electron propagator method (MCSTEP) ionization potentials: Application to methane, acetylene, ethylene, and ethane (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 5088-5093 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The multiconfigurational spin tensor electron propagator method (MCSTEP) is specifically designed to provide accurate determination of the low-lying principal vertical ionization potentials (IP's) and electron affinities (EA's) of atoms and molecules. In this paper we modify and extend our definition of an "optimal" small complete active space (CAS) for MCSTEP calculations. We then use MCSTEP to determine the vertical principal ionization potentials (IP's) below 19 eV of the small organic molecules methane, acetylene, ethylene, and ethane. The calculations are done using the standard Dunning-core-valence pVDZ and pVTZ basis sets. The results are compared with experiments as well as with some other forefront theoretical methods. The MCSTEP IP's are in very good to excellent agreement with experimental IP's. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474871
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  • 2
    Electronic Resource
    Electronic Resource
    Resolving the controversy over the second ionization potential of the nitrate free radical NO3 (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 4572-4578 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Two experimental studies determining the ionization potentials (IPs) of a nitrate free radical have recently been done. The first study performed using discharge flow-photoionization mass spectroscopy (PIMS) determined the adiabatic ionization potential at 12.57 eV. The second study using the HeI photoelectron spectroscopy (PES) observed five bands with the five lowest vertical ionization potentials of NO3 at 12.55, 13.18, 13.62, 14.05, and 15.54 eV. The second IP determined using PES (at 13.18 eV) has been questioned due to its absence in the spectrum obtained using the PIMS experiment. We use our theoretical method, the multiconfigurational spin tensor electron propagator method (MCSTEP), to help resolve the controversy over the ionization potential of NO3. We perform MCSTEP calculations on NO3 using the standard cc-pVDZ and cc-pVTZ basis sets at D3h and C2v geometries. Our results at both of these two symmetries support the experimental results obtained using PES. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481015
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  • 3
    Electronic Resource
    Electronic Resource
    Multiconfigurational spin tensor electron propagator vertical ionization potentials for O2: Comparison to some other forefront methods using the same basis sets and geometries (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 9444-9445 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In a recent paper in The Journal of Chemical Physics, we showed the potential energy curves for several cation states of O2 obtained using the multiconfigurational spin tensor electron propagator method (MCSTEP) with a 〈5s4p3d〉 basis set. In this communication we present vertical ionization potential calculations to the same O2 states. However, for the results reported here, exactly the same basis sets and geometries are used that were used for two other forefront methods; the coupled-cluster reference electron propagator theory (CC-EPT) and the Fock space multireference coupled-cluster method (FSMRCC). Hence, more direct comparisons and contrasts among these methods are now available. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468813
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  • 4
    Electronic Resource
    Electronic Resource
    Ionization potentials of CH2: A comparison of the multiconfigurational spin tensor electron propagator method with benchmark full configuration interaction and large scale multireference configuration interaction calculations (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 2947-2952 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Using the same basis sets and geometries as were previously used in "benchmark'' full configuration interaction (FCI) calculations we compare the multiconfigurational spin tensor electron propagator method (MCSTEP) with FCI for the vertical ionization potentials (IPs) in CH2 below 19.0 eV. Our results show that MCSTEP using a full valence complete active space MCSCF initial state accurately obtains the lowest several principal vertical ionization potentials. We also determine vertical and adiabatic IPs in CH2 with MCSTEP using larger bases and compare to accurate large scale multireference singles and doubles CI with quadruple excitations estimated via a Davidson correction.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466437
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  • 5
    Electronic Resource
    Electronic Resource
    Electron propagator method with a multiconfigurational second-order perturbation theory wave function as the initial state in the fermion operator block (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 5124-5136 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have developed an electron propagator method using a multiconfigurational second-order perturbation theory (CASPT2) wave function as the initial state [electron propagator CASPT2 (EPCASPT2)] in the fermion operator block (block 1). In the other blocks a multiconfigurational self-consistent field wave function is the initial state. We apply our new method to directly determine the low-lying vertical ionization potentials of Be, CH2, NH2, and H2O. We compare our results with the results of the calculations using multiconfigurational spin tensor electron propagator (MCSTEP), full configuration interaction (FCI), and multireference configuration interaction (MRCI) methods with the same geometries and basis sets. The calculations are performed using complete active space (CAS) choices that are usually excellent for MCSTEP ionization potential (IP) calculations and also for CAS choices that are inadequate for MCSTEP IP calculations. We show that EPCASPT2 generally improves MCSTEP IPs compared to ΔFCI when the MCSTEP IPs are in very good to excellent agreement with ΔFCI IPs and that EPCASPT2 can effectively mimic ΔFCI even when the CAS choice for the initial state is inadequate for MCSTEP. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1349077
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  • 6
    Electronic Resource
    Electronic Resource
    The multiconfigurational spin tensor electron propagator method (MCSTEP): Comparison with extended Koopmans' theorem results (1995)
    Springer
    Theoretical chemistry accounts 90 (1995), S. 273-290 
    Details
    ISSN: 1432-2234
    Keywords: Ionization ; Propagators ; Ionization potential ; MCSTEP ; Green's functions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary We applied the multiconfigurational spin tensor electron propagator method (MCSTEP) for determining the lowest few (in energy) vertical ionization potentials (IPs) of HF, H2O, NH3, CH4, N2, CO, HNC, HCN, C2H2, H2CO, and B2H6. We chose these molecules so that we could compare MCSTEP IPs with recently reported extended Koopmans' theorem (EKT) IPs on the same molecules. Using standard Dunning core-valence basis sets with relatively small complete active spaces, MCSTEP results are in very good to excellent agreement with experiment. These MCSTEP IPs are obtained using matrices no larger than 400 × 400. EKT matrices are even smaller; however, to obtain similar but generally slightly worse agreement with experiment, fairly large active spaces are required with EKT.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF01113536
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  • 7
    Electronic Resource
    Electronic Resource
    The multiconfigurational spin tensor electron propagator method (MCSTEP): Comparison with extended Koopmans’ theorem results (1995)
    Springer
    Theoretica chimica acta 90 (1995), S. 273-290 
    Details
    ISSN: 0040-5744
    Keywords: Key words: Ionization ; Propagators ; Ionization potential ; MCSTEP ; Green’s functions
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary.  We applied the multiconfigurational spin tensor electron propagator method (MCSTEP) for determining the lowest few (in energy) vertical ionization potentials (IPs) of HF, H2O, NH3, CH4, N2, CO, HNC, HCN, C2H2, H2CO, and B2H6. We chose these molecules so that we could compare MCSTEP IPs with recently reported extended Koopmans’ theorem (EKT) IPs on the same molecules. Using standard Dunning core-valence basis sets with relatively small complete active spaces, MCSTEP results are in very good to excellent agreement with experiment. These MCSTEP IPs are obtained using matrices no larger than 400×400. EKT matrices are even smaller; however, to obtain similar but generally slightly worse agreement with experiment, fairly large active spaces are required with EKT.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/s002140050072
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  • 8
    Electronic Resource
    Electronic Resource
    Balanced complete active space choices with the multiconfigurational spin tensor electron propagator method: The vertical ionization potentials of NH2 (1998)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 70 (1998), S. 729-736 
    Details
    ISSN: 0020-7608
    Keywords: ionization potentials ; electron propagator ; MCSTEP ; NH2 ; Green's function ; Chemistry ; Theoretical, Physical and Computational Chemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: With electron propagator methods, vertical ionization potentials (IPs) and electron affinities can be calculated directly. Our implementation, known as the multiconfigurational spin tensor electron propagator method (MCSTEP), is specifically designed for open-shell and highly correlated initial states. The initial state that is usually used in MCSTEP is a complete active space (CAS) MCSCF state. We have previously demonstrated that a small balanced CAS is the optimal choice for the MCSTEP initial state. In this article, we examine two ways of determining this balanced CAS for the MCSTEP initial state. With these choices, we calculate the low-lying vertical MCSTEP IPs of NH2 and compare them with experiment for the three lowest IPs. With (aug-)cc-pVTZ, (aug-)cc-pVQZ, and extrapolated complete basis set limits, the MCSTEP IPs for both CAS choices are in very good agreement with experiment. We also calculate the next two vertical IPs where no accurate experimental estimates are as yet available. The “SCF-balanced” CAS choice is advantageous to use with MCSTEP because of simplicity, while the “SDCI-balanced” CAS is appropriate for use with MCSTEP when the initial MCSCF has undesired properties.   © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 729-736, 1998
    Additional Material: 3 Tab.
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  • 9
    Electronic Resource
    Electronic Resource
    Comparison of perturbative and multiconfigurational electron propagator methods (1996)
    New York, NY : Wiley-Blackwell
    International Journal of Quantum Chemistry 60 (1996), S. 29-36 
    Details
    ISSN: 0020-7608
    Keywords: Computational Chemistry and Molecular Modeling ; Atomic, Molecular and Optical Physics
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Ionization energies below 20 eV of 10 molecules calculated with electron propagator techniques employing Hartree-Fock orbitals and multiconfigurational self-consistent field orbitals are compared. Diagonal and nondiagonal self-energy approximations are used in the perturbative formalism. Three diagonal methods based on second- and third-order self-energy terms, all known as the outer valence Green's function, are discussed. A procedure for selecting the most reliable of these three versions for a given calculation is tested. Results with a polarized, triple ζ basis produce root mean square errors with respect to experiment of approximately 0.3 eV. Use of the selection procedure has a slight influence on the quality of the results. A related, nondiagonal method, known as ADC(3), performs infinite-order summations on several types of self-energy contributions, is complete through third-order, and produces similar accuracy. These results are compared to ionization energies calculated with the multiconfigurational spin-tensor electron propagator method. Complete active space wave functions or close approximations constitute the reference states. Simple field operators and transfer operators pertaining to the active space define the operator manifold. With the same basis sets, these methods produce ionization energies with accuracy that is comparable to that of the perturbative techniques. © 1996 John Wiley & Sons, Inc.
    Additional Material: 1 Ill.
    Type of Medium: Electronic Resource
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